Optimal power allocation (PA) scheme for distributed antenna systems with orthogonal frequency division multiplexing (DAS-OFDM) over frequency-selective fading channels is presented. The PA design is to optimise the transmit powers of subcarriers to maximise the capacity subject to the total power constraint. Based on the Karush–Kuhn–Tucker optimality conditions, the optimal PA to the distributed nodes for a subcarrier can be simplified to allocating the power to the node of the largest channel gain. With this result, an efficient algorithm is derived to obtain the optimal PA to the nodes for all the subcarriers of the DAS-OFDM. Compared with an existing suboptimal scheme, the developed scheme can obtain higher capacity and has lower complexity. By computer simulations, the effectiveness of the proposed scheme is verified.